The present invention relates to a method and apparatus for spectral analysis of images to determine the thickness of a thin film, and particularly for spatially resolving film thicknesses of a coating deposited over the surface of a silicon wafer or other similar materials (for example, a flat panel display).
Measuring film thickness by reflectance spectroscopy is well known: see for example P. S. Hauge, "Polycrystalline silicon film thickness measurement from analysis of visible reflectance spectra", J. Opt. Soc. Am., No. 8, August 1979, and the book by Milton Ohring: "The material science of thin films", Academic Press Ltd., 1992. In today's microelectronic device manufacturing processes, the uniformity of the deposited films over a wafer is gaining importance as time goes on, because a good uniformity insures identity among the finished product chips. The size of the chips is also decreasing, so that the uniformity tolerance is becoming stricter. In addition, to insure high yield (low rejects) and efficient (low cost) manufacturing, the wafer inspection requires higher automation, shorter time, higher accuracy, and wider thickness range.
As a result, the film thickness map of the wafer, as one of the many inspections done during the manufacturing, must be done accurately, fast, on a large number of points (test sites), and at a wide thickness range.
Today, film thickness mapping instruments are based on ellipsometry or on reflectance spectroscopy. Only the latter is addressed herein. As examples of prior art in this field we mention the SpectraMap SM-300 and the FT-500 of Prometrix. The spectra are measured point by point by moving the wafer on a translation stage, in order to complete one thickness map. This takes time, it requires high movement accuracy, because of the high spatial resolution required, and increases the wafer handling, which is practical only when the wafer is outside a deposition chamber (therefore it cannot be done in-situ). In fact, the fastest thickness mapping mentioned by present manufacturers of film thickness equipment is hundreds of points in a few seconds.
There is thus a recognized need for, and it would be highly advantageous to have a method and apparatus for determining the spatial distribution of the thickness of a film overlying a substrate, more quickly, with higher spatial resolution (more test sites), without the need to move the wafer with respect to the measuring instrument when going from a test site to another (higher accuracy, and less wafer handling with the potential for in-situ monitoring), and easily measure the widest thickness range possible.
The present invention relates to a method and apparatus for mapping film thickness on Silicon wafers or similar substrates, which does not require moving the wafer (making the results faster and spatially more accurate, and potentially capable of being done in-situ), reaching tens of thousands of pixels in a few seconds (not hundreds as stated in the present commercial literature), and which has the potential to measure, in the same time as other potentially competing methods (mentioned below), a wider thickness range.
A spectrometer is an apparatus designed to accept light, to separate (disperse) it into its component wavelengths, and detect the spectrum. An imaging spectrometer is one which collects incident light from a scene and analyzes it to determine the spectral intensity of each pixel thereof.
The former measures the spectrum only at one point, therefore, with such an instrument, the wafer must be moved point by point relative to the instrument, and will have the above mentioned drawbacks of long measurement time and position accuracy.
The latter, i.e., an imaging spectrometer or spectral imager, can be of different types: a technology similar to the one used for resource mapping of the earth surface from airplanes and satellites could be used for film thickness mapping (see, for example, J. B. Wellman, Imaging Spectrometers for Terrestrial and Planetary Remote Sensing, SPIE Proceedings, Vol.: 750, p. 140 (1987)). However, this technique is based on grating technology to spectrally disperse the light: this brings the following drawback.
A grating has higher order diffraction: in order for it to be useful for spectral measurements, its spectral range must be limited by blocking the wavelengths outside a so called "octave" of wavelengths, for example 0.4 to 0.8.mu.; therefore an instrument based on a grating cannot have a wavelength range wider than one in which the ratio between the higher and the lower wavelength is larger than 2. This problem can be solved by measuring separately different octaves by rotating the grating to different angles, or by using several gratings simultaneously. However, these solutions increase the measurement time or complicate the instrument optics. The same problem is encountered by liquid crystal and acousto-optic crystal tunable filters. The importance of the wavelength range is related to the film thickness range to be measured by the instrument. In fact, as is well known in the optics literature, high precision film thickness measurements are difficult to obtain by reflection spectroscopy when the thickness t of the film is less than (.lambda./4)n, where .lambda. is the minimum wavelength of the instrument sensitivity range, and n is the refractive index of the film. Therefore, the wider the wavelength range to which the instrument is sensitive, the wider the thickness range that it can measure.
It is also well known (see, for example, the book R. J. Bell, "Introductory Fourier Transform Spectroscopy", Academic Press 1972), that interferometers do not have this limitation, and therefore can more easily measure a wider range of thicknesses.
Our U.S. Pat. No. 5,539,571, which is incorporated by reference in its entirety for all purposes as if fully set forth herein, discloses a method of analyzing an optical image of a scene to determine the spectral intensity of each pixel of the scene, which includes collecting incident light from the scene, passing the light through an interferometer which outputs modulated light corresponding to a predetermined set of linear combinations of the spectral intensity of the light emitted from each pixel; scanning the light beam entering the interferometer with respect to the interferometer or scanning the interferometer itself, to scan the optical path difference (OPD) generated in it for all the pixels of the scene separately and simultaneously; focusing the light outputted from the interferometer on a detector array, and processing the output of the detector array to determine the spectral intensity of each pixel thereof.
The above-referenced patent application provides a method and apparatus for spectral analysis of images which better utilizes all the information available from the collected incident light of the image to substantially decrease the film thickness measurement time, reach higher spatial resolution, and increase thickness dynamic range, as compared to other existing instrumentation. From here we can see how our method can give a thickness map of tens of thousands of pixels in a few seconds. Presently available detector matrices have 16,384 (128.times.128) pixels which are scanned at 1,000 frames per second (16 MHz). As a consequence, since each frame gives the separate and simultaneous information about the OPD of every pixel of the image (which is equivalent to the spectral information by Fourier Transform), at the end of a second all the needed spectral information is collected for all those pixels.